The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of the IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC manufacturing are needed.
For example, extreme ultraviolet (EUV) lithography has been utilized to support critical dimension (CD) requirements of smaller devices. EUV lithography employs scanners using radiation in the EUV region, having a wavelength of about 1-100 nm. Some EUV scanners provide 4× reduction projection printing, similar to some optical scanners, except that the EUV scanners use reflective rather than refractive optics, e.g., mirrors instead of lenses. Masks used in EUV lithography present new challenges. For example, an EUV mask generally uses a multi-layer (ML) structure over a substrate and a microscopic non-flatness of the substrate may deform the ML structure deposited subsequently thereon. When an incident EUV radiation is reflected from a deformed region, it may experience a phase difference with respect to a radiation reflected from a normally formed region. Such mask defects are commonly referred to as phase defects. A phase defect may affect print fidelity and result in reduced image intensity or even pattern distortion on a wafer. Certain compensation methods, such as requiring tighter process controls for mask production or repairing mask defects, may be utilized. These compensation methods, however, may significantly increase mask fabrication time and cost. Accordingly, although existing lithography methods have been generally adequate, they have not been satisfactory in all respects.